It is very much worth it. I've been using it for over a year now and love it. I used to shy away from these types of rad tools, but it has changed the way I work. With the 1.2 release it has gotten even better with better stack trace info (particularly for GSPs).
The only problem I have had with scaling has been hibernate and it's cache, which really isn't a grails issue. Even those I don't like hibernate in general, the way grails wraps it with GORM is a work of art to me. The criteria closure aspect is wonderful to work with.
